Building on the successes of our Protein Reviews on the Web (PROW), we are now developing a unique resource for the biological community which provides information on phosphorylation sites on human proteins. That site, MPR ("Mammalian Phosphorylation Resource") is now publicly available (http://mpr.nci.nih.gov/mpr ) as a pilot project to use human proteins as the organizing principle around which to build a conceptual web of biological concepts. Although it is in its infancy, already it is the most complete and robust sequenced-based annotation of human phosphorylation sites. MPR contains information on more than 400 antibodies directed against 311 sites on proteins encoded by 156 genes. MPR is unique among web resources. Of particular importance is the fact that MPR includes information on the amino acid sequence surrounding the site. Given the existing problems with gene nomenclature and with specification of residue number (caused by alternate splicing and mixing numbering from different species), such sequence information is the only simple way to unambiguously specify the site. Since such sequence information is often not supplied in association with antibody information, accumulation and validation of this sequence information has been a critical and substantial task in establishing MPR. Another unique feature of the site is the ability to use BLAST searches for finding the phosphorylation sites closest to a submitted query sequence. Work has started on tools to facilitate the process by which a curator specifies the gene and sequence information of a phosphorylation site he/she is adding to MPR. We expect to set this up in such a way that antibody providers can efficiently provide information to MPR and thereby keep the content of MPR as current and complete as possible